Electrostatic spraying

ABSTRACT

In an electrostatic powder spraying apparatus, powder is conveyed through a dielectric tube to a corona discharge device by vibratory motion of the tube. The powder is dispersed in a cloud downstream of the corona discharge device by a vibrating member at the end of the tube.

United States Patent Field of Search 239/3, 15; 222/199, 200; i 117/93.4 R;317/3 Legge et a1. Mar. 18, 1975 ELECTROSTATIC SPRAYING [75] Inventors: Morris James Legge, Farnham; [56] References C'ted 19h! Ab teirii t rit iliert qettzhs; UNITED STATES PATENTS Roger Patrick Corbett, 2,550,852 12/1970 Southampton, all of England ,38 ,050 5/1 68 3,496,911 2/1970 [73] Assignees: Plastic Coatings Limited, Surrey, 3558952 H1971 Y England; University of 3,637,135 1/1972 Luderer et a1. 239/3 Southampton, Southampton, v England Primary Examiner-Robert S. Ward, Jr 7 Assistant Examiner-Michae1 Mar [22] Flled' 1973 Attorney, Agent, or FirmLarson, Taylor & Hinds [21] v Appl. No.: 386,618

[57] ABSTRACT [30] Foreign Application Priority Data i an electcrlosltlatic i gs p qy pp i p g is conveye t roug a te ectrlc tu e to a corona 15- 1972 Umted Kmgdom charge device by vibratory motion of the tube. The [52] U S Cl 239/3 239/15 317/3 powder is dispersed in a cloud downstream of the co- [51] Bosb 5/02 rona discharge device by a vibrating member at the [58] end of the tube.

7 Claims, 3 Drawing Figures ELECTROSTATIC SPRAYING This invention relates to an apparatus for electrostatic powder spraying and to a method of electrostatic powder spraying.

According to one aspect of the present invention there is provided an apparatus for electrostatic powder spraying comprising a corona discharge device and a powder dispenser for conveying powder to the corona discharge device characterised in that the powder is conveyed along or through the dispenser solely by vibratory motion of the dispenser.

Preferably the dispenser comprises a dielectric tube having an open end for discharge and an inlet for receiving powder adjacent its opposite end, and means for vibrating the tube whereby powder fed to the tube is caused to flow along the tube to the corona discharge device.

Preferably there is further provided a vibratable member for dispensing the powder in a cloud in the vicinity of the corona discharge device.

The vibratable member preferably comprises a plate adjacent the open end of the tube and vibratable in a plane substantially at right angles to the longitudinal axis of the tube.

The vibratable member and the corona discharge device are preferably located on an extension member attached to the open end of the tube, the vibration of the vibratable member being provided by a component of the vibration of the tube.

Part of the corona discharge device may provide the vibratable member.

The tube may be vibrated at a frequency of between and 200 Hz, preferablySO Hz at an amplitude of approximately 1 mm.

The dispenser may be located above and separately from the corona discharge device so that powder falling from the dispenser past the discharge device becomes electrostatically charged.

According to another aspect of the invention there is provided a method of electrostatic powder spraying comprising supplying a stream of powder particles to a corona discharge device solely by vibrating them along a vibrating dielectric tube and dispersing them in a cloud downstream of the corona discharge device by means of a vibrating member.

The invention will now be described by way of example with reference to the diagrammatic accompanying drawings in which FIG. 1 is a side elevation of an electrostatic powder spraying apparatus according to the invention,

FIG. 2 is a plan view of a discharge portion of the apparatus shown in FIG. 1,

FIG. 3 is a diagrammatic representation of an electrostatic powder spraying apparatus according to another aspect of the invention.

Referring to the drawings there is shown an electrostatic spraying apparatus, or spray gun, comprising a vibrator tube of dielectric material, e.g., Perspex (Reg. Trade Mark), of about 1 inches internal diameter, which is clamped by means of clamps ll, 12 to a vibrator machine 13. The dispositionof the clamps 11, I2 and the vibrator machine 13 are such that the tube 10 vibrates along its longitudinal axis in the directions indicated by the arrow 14, and because of inherent tolerances in the mechanism, also has a component of vibration at right angles to its longitudinal axis.

In the Perspex tube 10, near one'end, is an inlet tube 15 through which powder may be fed in the direction shown by arrow 16.

Held to the other, open, end of the tube 10 by a clamp 17 is an extension member provided by a divergent guide plate 18 downstream 18 to the of which is attached a corona discharge element 19 and a vibratable member in the form of a thin brass flap 20 which extends in line with the tube 10.

The corona discharge element is maintained at a high potential by means of a high tension voltage supply line 21 fed from a high voltage source 9.

In a non-limiting example, epoxy resin powder of a particle size such that 50 percent is less than 50a is gravity fed into the tube 10 by way of the supply tube 15. The tube 10 is held in a horizontal position, and is vibrated by the vibrator 13 at a frequency of 50 Hz and at a longitudinal amplitude of about 1 mm. This vibration causes the powder in the tube to move in the direction of the arrow 22 onto the guide plate 18.

The particles are charged partly by contact with the guide plate 18, which is maintained at a potential of about 48 kV, and mainly by ion bombardment in the region in front of the discharge element 19. They are then accelerated by the electric field created between the discharge element 19 and the earthed object 23 to be coated by the powder, and are propelled on to this object 23.

The vertical component of vibration of the Perspex tube 10 causes the brass flap 20 to vibrate sympathetically in a plane perpendicular to the plane of vibration of the tube 10 as indicated by the arrows 24. The effect of the vibrating flap 20 is to disperse the powder in a cloud and enable the powder particles more readily to take up their electrostatic charge.

In this example, typical rates of flow of the powder through the spray gun described above may vary between 27 and 123 grammes per minute.

In another embodiment of the invention, shown in FIG. 3, the powder 33 is dispensed from a grounded metal vibratory dispenser 30, which is maintained well clear of the charging electrode 31, and is allowed to fall past the charging electrode where it is charged and deposited on the object 32 to be coated.

It is thus seen that the invention provides an apparatus for and a method of airless electrostatic spraying of powder, airless in the context of the present invention being understood to mean that a moving stream of air or other fluid is not used as the means for conveying electrostatically charged powder particles to an object to be sprayed.

An advantage of the present invention over known electrostatic powder spray guns in which the powder is carried to the article to be coated partly by an air or other fluid stream is that the need for an air supply is eliminated and the danger of particles being carried in the air stream beyond the article to be coated is minimized. Furthermore, a flow of air is a possible source of powder contamination (oil or water) and is an additional variable when consistency of coating thickness is the aim. Consequently, the airless spray gun of the present invention is more efficient in that it is capable of depositing a larger proportion of its through-put of powder on the object to be coated than other guns so far commercially available.

In an alternative embodiment the corona discharge element 19 may be omitted and the high tension voltage supply line 21 attached directly to the brass flap 20 which then generates the required electrostatic field.

The flap 20 may be made of a material other than brass provided it is able to vibrate sufficiently to disperse the powder stream in a cloud of particles.

The frequency at which the vibrator operates may suitably lie in the range 5 to 200 Hz.

The tube may be replaced by an open channel.

What we claim is:

1. An apparatus for airless electrostatic powder spraying comprising a corona discharge device, a vibratory dielectric hollow elongate means for conveying powder to the corona discharge device, the elongate means having an open end for delivering powder to the corona discharge device and an inlet for receiving powder adjacent its opposite end, means for vibrating the elongate means such that powder fed to the elongate means is caused to flow therealong to the corona discharge device, and a vibratable member for dispersing the powder in a cloud in the vicinity of the corona discharge device.

2. An apparatus as claimed in claim 1 wherein the elongate means comprises a tube.

3. An apparatus as claimed in claim 1 wherein the elongate means comprises an open channel.

4. An apparatus as claimed in claim 1 wherein the vibratable membercomprises a plate adjacent the open end of the elongate means and vibratable in a plane substantially at right angles to the longitudinal axis of the elongate means.

5. An apparatus as claimed in claim 1 wherein the vibratable member and the corona discharge device are located on an extension member attached to the open end of the elongate means, the vibration of the vibratable member being provided by a component of the vibration of the elongate means.

6. An apparatus as claimed in claim 1 wherein part of the corona discharge device provides the vibratable member.

7. A method of electrostatic powder spraying comprising supplying a stream of powder particles to a corona discharge device solely by vibrating them along a vibrating dielectric tube and dispersing them in a cloud downstream of the corona discharge device by means of a vibrating member. 

1. An apparatus for airless electrostatic powder spraying comprising a corona discharge device, a vibratory dielectric hollow elongate means for conveying powder to the corona discharge device, the elongate means having an open end for delivering powder to the corona discharge device and an inlet for receiving powder adjacent its opposite end, means for vibrating the elongate means such that powder fed to the elongate means is caused to flow therealong to the corona discharge device, and a vibratable member for dispersing the powder in a cloud in the vicinity of the corona discharge device.
 2. An apparatus as claimed in claim 1 wherein the elongate means comprises a tube.
 3. An apparatus as claimed in claim 1 wherein the elongate means comprises an open channel.
 4. An apparatus as claimed in claim 1 wherein the vibratable member comprises a plate adjacent the open end of the elongate means and vibratable in a plane substantially at right angles to the longitudinal axis of the elongate means.
 5. An apparatus as claimed in claim 1 wherein the vibratable member and the corona discharge device are located on an extension member attached to the open end of the elongate means, the vibration of the vibratable member being provided by a component of the vibration of the elongate means.
 6. An apparatus as claimed in claim 1 wherein part of the corona discharge device provides the vibratable member.
 7. A method of electrostatic powder spraying comprising supplying a stream of powder particles to a corona discharge device solely by vibrating them along a vibrating dielectric tube and dispersing them in a cloud downstream of the corona discharge device by means of a vibrating member. 